Accessibility of geometrically-rough (fractal) surfaces of natural sorbents to probe molecules

Vapor sorption of a series of probe molecules was studied to investigate the accessibility of surfaces for adsorption. The hypothesis was that the amount of probe molecules required to cover a surface with a monolayer (i.e., monolayer value, Nm, mmoles/g sorbent) is affected by the molecular level roughness of the surface. In particular, the applicability of the concept of fractal dimension was considered as the conceptual guideline. The experimental results indicated that the surface structure of simple sorbents, such as kaolinite and silica-gel, can be interpreted as fractal-like. However, several factors severely limited the accurate estimation of the fractal dimension from sorption data; these include 1) unknown surface configuration/packing of adsorbed molecule, 2) experimental difficulty (low vapor sorption) associated with large probe molecules, 3) unrealistic assumptions inherent in vapor sorption theory (B.E.T. theory), and 4) slow sorption/desorption kinetics and hysteresis. The result for Borden sand showed that sorption of various probe molecules by this medium could not be explained from fractal theory. For such a chemically heterogeneous natural system, the chemical nature (hydrophilicity) of the probe molecule seems to strongly influence the accessibility of the sorption domain to the probe molecule.